Drilling on the relief well has stopped while BP prepares to raise the failed blowout preventer from the sea floor. The effort is not just a safety precaution. The equipment is also evidence that may reveal how the Gulf oil spill happened.

In this video still released by BP, oil can be seen pouring out of several spots near the blowout preventer on June 3. BP is preparing to raise the failed blowout preventer from the sea floor, so that it can be used as evidence in a Justice Department investigation into the accident.

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The 450-ton blowout preventer resting a mile below the surface of the Gulf of Mexico is set to become Exhibit A in a Justice Department investigation into what caused the explosion aboard the Deepwater Horizon rig, killing 11 men, sinking the platform, and resulting in the biggest oil spill in US history.

Preparations are under way for the massive blowout preventer stack of valves – a unit taller than a school bus is long – to be brought to the surface. The piece of equipment, says Robert Bea, a former engineer for Shell Oil who is now a professor at the University of California at Berkeley, is "a key part of the crime scene."

Since the disaster began in April, the public has been bombarded with detailed explanations about how the "shear ram" valve on the blowout preventer (BOP) was supposed to slice through the drill pipe in an emergency and shut off the well. That it failed is clear enough.

But why it failed remains anything but clear. All the digging through documents, all the mountains of testimony before Congress and federal panels have led only to conjecture about possible causes of failure. Careful dissection of the BOP and a look inside the steel jaws that did not close now appear to be the only ways to learn what happened.

"The BOP is this key piece of physical evidence – perhaps the last best hope for putting the pieces of the puzzle of the Deepwater Horizon together," says John Rogers Smith, associate professor of petroleum engineering at Louisiana State University.

Thad Allen, the Gulf national incident commander, on Thursday gave the go-ahead for BP and Transocean, the drill rig operator, to replace the Deepwater Horizon's BOP with another and to bring the failed unit to the surface. But he warned BP that the BOP must be treated carefully to preserve evidence.

"Each procedure should recognize and preserve the forensic and evidentiary value of the BOP and any material removed from the BOP," he wrote in Thursday's directive to BP Chief Managing Director Bob Dudley. Along those lines, Justice Department investigators are now hunting for blowout preventer specialists who could conduct a forensic examination of the unit, one source told the Monitor.

The BOP may be the last best chance to find a "smoking gun" that tells investigators whether the blowout was a result of human error, mechanical failure, bad maintenance, faulty procedures – or a combination of those, industry experts say.

"Looking carefully at that BOP is going to be critical to understanding what happened," agrees Adam Bourgoyne, a BOP expert and former dean of the College of Engineering at Louisiana State University. "With it, you're going to be able to analyze a lot about what happened just prior to the explosion."

For instance, there's the question of whether the BOP was opened and closed multiple times in the confusion of the blowout after high-pressure gas started shooting across the deck of the Deepwater Horizon rig. That might explain why, in video images, two pieces of pipe appeared to be sticking out of the top of the BOP. If the unit makes it to the surface with the pipe still inside it, part of the mystery could be solved.

"The BOP could have closed, once shut off at the sea floor," he explains. "But with all the expanding oil and gas still flowing to the surface a mile above, there could have been confusion aboard the rig over whether it actually closed or not – and the operators might have tried it again."

Dan Albers, a petroleum engineer and member of the Deepwater Horizon Study Group at UC Berkeley, says the BOP could help answer questions about a major theory concerning the device's malfunction.

If oil and gas shot up the gap, or "annulus," between the rock and the drill casing (a steel pipe just over nine inches wide), it could have lifted that large-diameter pipe and jammed it up into the vicinity of the BOP shear rams. BP never installed the casing hanger lockdown device, Mr. Albers says. If that happened, it would have made it impossible for the blind rams to close.

"This still doesn't answer the questions as to why the other BOP devices didn't activate," he writes in an e-mail. "These I relate to electro hydraulic control problems caused when the well blew out and the fire ensued destroying the control system."

Getting the BOP to the surface intact is key to determining whether such theories hold water, Dr. Bea writes in an e-mail interview.

"If the salvage of the BOP and the previous 'activities' have not destroyed the evidence that would confirm that the casing was shoved upward, then we would have one most plausible scenario in the drilling that led to the blowout," he says.

Mr. Allen said Thursday that BP had been directed to flush out the current blowout preventer and capping stack, clean it, and fill it with sea water. After that would come pressure tests. If those tests show the cement cap is holding, then the BOP could be removed and replaced by another BOP now being used by the second relief well from the Development Driller II rig. This process is expected to take place the week after Labor Day.

The main reason for doing this, Allen said, is to put the best possible BOP on the well in advance of pumping heavy mud into the bottom of BP's Macondo well through the adjacent primary relief well. It's a safeguard just in case any weaknesses remain in the concrete cap already put in place from the top last month.

While the tests go on, drilling of the relief well has been halted just 50 feet above and three feet to the side of the main well. This is being done out of "an overabundance of caution related to minimizing risk associated with the intersection of the well," Allen told reporters.

That said, each step would also be viewed through another lens: court. To that end, he said, the BOP would be protected.

"We do not want to have damage to that blowout preventer if we can avoid it," Allen said, "because it's going to be material evidence to exactly what happened during the event itself."